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INTRODUCTION TO SEMICONDUCTOR TECHNOLOGY

2.3 HOW DOES A TRANSISTOR WORK?

Transistor is the basic element of an MCU device. There can be hundreds of thousands of

them and the size of their gate can go down to 0.35 microns.

Let’s explain the way a NMOS transistor operates. Basically, There’s a lack of electrons be-

tween the Source (S) and the Drain (D) because this area has been implanted with a P-type

dopant (Boron for instance). Therefore, when no voltage is applied, there’s no current between

the source and the drain (case of the enhancement transistor).

If we apply a positive voltage on the gate and the drain, then this will attract electrons in the

channel existing between the source and the drain, therefore making it possible for an elec-

trical current to flow between S and D.

There are two main types of transistor:

– Enhancement Transistor: channel is permanently OFF. It requires a positive applied gate

voltage to turn on. Microcontrollers, for instance, mainly use this type of transistor.

– Depletion Transistor: channel is permanently ON. It requires a negative applied gate volt-

age to turn off.

This scheme is a cross section of

a real transistor obtained after

about 100 steps of fabrication

(see previous paragraph for the

explanation of the different fabri-

cation steps). The last layer is

called a passivation layer and pro-

tects the transistor.